If you encouter "unknown error (3002)", you probably do not have your ECID SHSH's for 3.0 "on file" with Cydia. Unfortunately, as Apple is no longer allowing users to sign the 3.0 firmware, it is no longer possible to register your device with Cydia.
Luckily, it has been reported that iPhoneOS 3.1 is vulnerable to another exploit. This means that, once a jailbreak is released for 3.1, users will be able to prepare themselves for future jailbreaks even if they missed the first round of signature storage (which I unfortunately was only able to start very late in the 3.0 game).
Once you even attempt to use this service (or if you tell Cydia to "make your life easier") you will be signed up for the signature tracker, and Cydia HQ will do its best to manage your ability to restore.